feat: switch the mermaid renderer to AlexanderGrooff/mermaid-ascii

[hidekingerz]

Summary

Replaces the mermaid renderer. pgavlin/mermaid-ascii is an unmaintained fork (issues disabled, last push March) whose flowchart renderer places text byte by byte, so a Japanese node or edge label came out as mojibake and #25 had to fall back to source. AlexanderGrooff/mermaid-ascii is the maintained original (1.5k stars, monthly releases) and measures text by display width; flowcharts with wide characters draw correctly.

• Its rendering entry point lived in the cmd package next to cobra and the gin web server, which would have added ~10 MB. A fork moves the graph renderer to pkg/graph and RenderDiagram to pkg/render, proposed upstream as Move graph rendering and RenderDiagram out of cmd into pkg/graph and pkg/render AlexanderGrooff/mermaid-ascii#94; go.mod replaces the module with hidekingerz/mermaid-ascii@fc194f1 until that lands. The binary is 0.7 MB smaller than with the old library.
• internal/ui/mermaid.go calls render.RenderDiagram; the non-ASCII flowchart fallback from fix: show a flowchart with non-ASCII text as source instead of mojibake #25 is removed.
• Trade-off: class and state diagrams are not supported by this library and now fall back to source (the old one drew state diagrams badly, which README already called a weak spot). A decision node B{…} is drawn as a plain box. README (both languages) updated.
• Sequence diagrams with wide characters in messages still misalign the lifelines; upstream has an open fix (Fix sequence diagram CJK/Unicode character rendering AlexanderGrooff/mermaid-ascii#55).

Test plan

• gofmt -l ., go vet ./..., TMPDIR=/tmp go test -race ./... clean
• New test: a flowchart with Japanese node and edge labels draws, the labels are intact, and every row of the drawing has the same width (wide characters counted as two columns)
• Existing mermaid tests unchanged and passing: fence replacement, fallback for undrawable and too-wide blocks, exact-width fit through glamour, mermaid = false, logrus silenced
• tmux: Japanese flowchart with branches and labels, and a subgraph flowchart, drawn correctly in reader mode

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https://claude.ai/code/session_01KUYF9hXeZFML1s8r3i2RVX